In the current state of the art, a brake-assist device contains an electric servomotor having a ball-screw assembly that transforms a rotational movement of the ball forming the rotor of the electric motor into a translational movement of the screw that moves a moving part in the direction of a master cylinder. The servomotor is activated by a control rod moved by a brake pedal. A force sensor measures the force likely to be applied to the control rod by the driver through the brake pedal.
The pressure in the master cylinder changes as a function of the force exercised on the brake pedal, the so-called braking force.
Initially, the braking force is intended to overcome the resistance of a return spring of a plunger piston in rest position. When the servomotor is at rest, the pressure in the master cylinder remains constant. As the braking force increases, the resistance of the plunger piston return spring is overcome and the servomotor is activated. However, when the servomotor is in its rest position, axial play is present between the plunger piston, alone or equipped with a sensor, and the moving part. As a result, activation of the servomotor provokes a pressure jump in the master cylinder when there is constant force. It can be seen, therefore, that the greater the pressure increase in the master cylinder, the more the action on the brake pedal is felt as being effective by the driver.
Information about control rod travel is sent to a control unit, which, on the basis of this information, generates a command to the electric motor that moves the moving part in correspondence with the force exercised on the control rod.
The known device can be used to regulate axial play by varying the differential travel of the plunger piston with respect to the moving part as a function of the driver's driving mode.
There are, in practice, several categories of drivers. Some drive rather aggressively, and we can qualify this mode as sporty driving; others drive with greater restraint, and we can qualify this mode as relaxed driving. However, with a device such as that known to the state of the art, it is not possible to differentiate a braking force applied to the brake pedal by a driver who prefers a sporty driving mode from a driver who prefers a relaxed driving mode. This drawback is aggravated by the fact that a driver who prefers a relaxed driving mode must exert a force on the brake pedal similar to that of a driver who prefers a sporty driving mode to begin braking the vehicle, and vice versa.